1. Field of the Invention
The present invention relates to alumina sintered bodies and base substrates for optical devices.
2. Description of the Related Art
Sintered polycrystalline alumina (Al2O3) bodies have been widely used as materials with good mechanical strength, insulation, thermal shock resistance, and corrosion resistance. Sintered polycrystalline alumina bodies are known to exhibit improved mechanical strength, thermal shock resistance, and corrosion resistance when the crystal orientations of the constituent grains are aligned in a particular direction (i.e., oriented) by controlling the microstructure. This is because such alumina sintered bodies show anisotropy in fracture toughness, dielectric constant, thermal conductivity, and thermal expansion coefficient resulting from the crystal orientation. For example, PTL 1 discloses that an oriented alumina sintered body prepared using a plate-like alumina powder as one ingredient has good heat resistance and corrosion resistance. In PTL 1, a compact prepared using a plate-like alumina powder as one ingredient is fired; however, the resulting compact will not be sufficiently oriented since the alumina purity is low, i.e., less than 99.9% by mass, and the compact is fired at low temperature, i.e., 1,500° C. to 1,750° C., by pressureless sintering. In addition, the resulting oriented alumina sintered body will not be transparent since PTL 1 does not mention optical translucency.
High-density sintered polycrystalline alumina bodies are known to be optical translucent at reduced impurity concentrations and are used for applications such as high-pressure sodium lamp tubes, high-heat-resistance window materials, components for semiconductor devices, and substrates for optical components. Although several attempts have been made to improve the optical translucency of optical translucent alumina sintered bodies, including controlling the crystal grain size and reducing pores and impurity concentration, sufficient optical translucency has yet to be achieved. One possible cause is birefringence resulting from the crystal structure. Alumina sintered bodies are known to have optical anisotropy resulting from the crystal structure and exhibit limited optical translucency if the polycrystalline alumina has a low degree of orientation. For example, in NPL 1, an oriented sintered polycrystalline alumina compact is prepared by a combination of slip casting and magnetic orientation to achieve improved optical transparency.